1. Field of the Invention
The present invention relates to a cam shaft support apparatus for an engine, and more particularly, to a cam shaft supporting structure for a DOHC engine including a pair of cam shafts for opening or closing suction valves and exhaust valves for each of a number of cylinders.
2. Description of the Prior Art
In a DOHC, i.e. double overhead cam shaft, type engine for use in an automobile, one cam shaft for suction valves and another cam shaft for exhaust valves are provided on a cylinder head and are interlocked by a rotation force transmission means. By inputting an engine rotation force to either one of the pair of cam shafts, these shafts are rotated in synchronization with each other so as to open and close respective suction valves and exhaust valves by means of swing crank arms.
This type of DOHC engine, wherein a cam shaft for suction valves and a cam shaft for exhaust valves are arranged in parallel on a cylinder head, is generally known, for example, in Japanese Utility Model Application Publication No. Sho 61-171801.
For example, as shown in FIG. 13, in a swing arm cam shaft support structure of a conventional DOHC engine of the typing having two suction valves and two exhaust valves as viewed in the longitudinal direction (in FIG. 13, in the right & left direction) of the cylinder head 201, a suction valve side bearing 202 is arranged between two HLA bosses 203 for suction valves, namely between two suction valve guides 204. By this bearing 202 and a cap (not shown) tightened by bolts onto the upper surface of the bearing 202, the suction valve side cam shaft (not shown) is rotatively supported. Similarly, an exhaust valve side bearing 205 is arranged between two HLA boss portion 206 for exhaust valves, namely, between two exhaust valve guides 207.
Furthermore, as shown in FIG. 14, in a direct type of support structure of a conventional two suction valve and two exhaust valve type of engine, viewed in the longitudinal direction (in FIG. 14, right and left direction) of the cylinder head 208, a suction valve side bearing 209 is arranged between two HLA boss portions 210 for suction valves, and an exhaust valve side bearing 211 is arranged between two HLA boss portions 212 for exhaust valves.
In the above-described conventional cam shaft support structure, in order to reduce the sliding resistance between the cam shaft and the bearing portion, the thickness in the axial direction of the bearing is generally reduced at the lower portion thereof.
In the conventional bearing portion, however, since oil supplied to the bearing portion reaches only part of the sliding surface of the bearing and a sufficient oil film is not formed, there is a problem in that the sliding resistance is not reduced so very much. Also, since the thickness in the axial direction of the bearing is reduced, there is a problem in that the P V value becomes high at a high rotation of the cam shaft, and the bearing reliability is reduced.
In addition, since the lubrication performance of oil is greatly reduce at an engine start operation under an extremely cold temperatures, it is necessary to set the clearance between the cam shaft and the bearing at a large value in consideration of this matter. For this reason, there is a problem in that the cam shaft support rigidity is reduced at an high speed rotation, and an abnormal behavior of the valve driving system takes place.
Furthermore, when a bending load is applied to the cam shaft, the cam shaft undergoes a bending deformation such that the cam shaft is projected radially approximately at a central portion of the bearing, and this projecting portion thereof causes a strong contact with the central portion of the bearing, namely so-called uneven contact. There is such a problem in that on the portion where an even contact takes place, there may take place bearing gall.
Although a counter measure of in achieving reduction of the sliding resistance by increasing the oil amount supplied to the bearing is conceivable against these problems, this will result in a problem in that the oil supply system becomes bulky, leading to a bulky size of the engine.